The present invention relates to optical encoders which detect a position, moving speed, and moving direction of a moving body using a photodiode as a photodetector, and more particularly to an optical encoder which is suitable for uses including printing apparatuses such as printers and copiers, and factory automation equipment by way of an example.
An example of a conventional optical encoder will be described by way of the optical encoder disclosed in JP 2001-99684 A.
As shown in FIG. 10, the optical encoder is provided with a light-emitting part (not shown) and a light-receiving part 302 across a moving body 301. The moving body 301 is provided with a plurality of slits 305 which are formed with a prescribed pitch and moves along a moving direction Z shown as arrow Z. The light-receiving part 302 receives lights which are emitted from the light-emitting part and transmitted through the slits 305 of the moving body 301. The light-receiving part 302 is provided with a plurality of photodiode groups, each of which is composed of four photodiodes 3061 to 3064. The photodiode groups arranged along the moving direction Z. The four photodiodes 3061 to 3064 face three slits 305 of the moving body 301.
When the moving body 301 moves along the moving direction Z relative to the light-emitting part and the light-receiving part 302, the light-receiving part 302 receives lights which are emitted from the light-emitting part and transmitted through the slits 305 and outputs four independent optical modulation signals, namely, movement signals A+, B+, A− and B− from four photodiodes 3061, 3062, 3063 and 3064, respectively.
When the optical encoder reads movement information of the moving body 301, the equal light quantity distribution on the light-receiving surface of the light-receiving part from a light source for the light-emitting part shown in property A of FIG. 7 is ideal. In this case, only the information of the moving body 301 is accurately read as an optical modulation signal.
In reality, however, lights which are incident on the light-receiving surface of the light-receiving part show a light quantity distribution like that in property B of FIG. 7. Examples of the reasons for this distribution are light quantity distribution variance from the light source itself; light quantity distribution variance caused by a converging lens of the light source; diffraction and refracted lights caused by a moving body or the like; irregularities in positional relationship caused by assembly irregularities such as parallelism of the moving body to the light-receiving surface; irregularities in the slit sizes of the moving body; irregularities in the light source, the moving body and the light-receiving surface caused by staining and the like; and light receiving sensitivity variance caused by the variation of the moving speed of the moving body.
Therefore, a plurality of the movement information signals obtained from the light-receiving part are subject to factors such as DC voltage offset between signals, amplitude fluctuation of signals, signal waveform distortion, and phase distortion among signals, so that the movement information obtained is made inaccurate.